Such devices for mixing a powder with conveying air are sufficiently known. In connection with this, compressed air, for example, is blown into a storage container, the powder stored therein is swirled and a part of the swirled powder is carried out. It is disadvantageous in connection with this device that the amount of powder carried out is greatly dependent on the fill state of the container. However, it is desired that a constant amount of powder be carried out permanently, or at least over a very long period of time, so that fluctuations in the application of the powder to printed sheets of paper are prevented. Moreover, with these devices, the powder-air mixture is difficult to vary, or respectively to adjust it to changing outside conditions.